Remove sha256 from the source's digest

[robaiken]

What are you trying to accomplish?

Fixing how Dependabot handles Docker image digest updates in Docker Compose by:

1. Modifying the regex pattern to properly parse sha256: prefixed digests
2. Storing only the digest hash (without sha256: prefix) in the dependency's source hash
3. Reusing Dependabot's existing Docker version updater logic for Docker Compose updates
4. Updated the digest parser in the Tag class to use the FileParser constant

Thanks to @gegoune #390 (comment) and @DigitallyRefined #390 (comment) for raising this issue

Anything you want to highlight for special attention from reviewers?

Removed custom update_digest_and_tag implementation to reuse the base Docker version updater

How will you know you've accomplished your goal?

The changes will be verified when:

1. Digest updates preserve the sha256: prefix in the final output
2. Version updates work correctly using Dependabot's Docker version updater
3. PR titles and descriptions show proper version information

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

[gegoune]

Thank you! Does it also mean that a human readable versions after hash character will be updated as well?

[markhallen]

[robaiken]

@gegoune When this gets deployed, Dependabot should shorten the hash to six chars, but I will not know until the rubber hits the road